Tandem rear axle assemblies having a forward rear axle and a rearward rear axle in proximity with each other are well known. Such tandem axle assemblies are widely used on trucks and other over-the-road motor vehicles, such as busses, which have a high vehicle weight and a high load carrying capacity. In such assemblies, both rear axles may be power driven.
An interaxle differential is commonly employed to divide power between drivetrains for the respective rear axles. An interaxle differential typically includes a single input shaft, which may be in direct drive relationship with a vehicle drive shaft, and gearing which includes a pair of output gears for delivering torque to the respective axle drivetrains. One output gear typically drives a shaft which drives the rearward rear axle for example, while the other output gear typically drives a pinion shaft which in turn drives the forward rear axle. Under normal operating conditions, i.e., when the wheels mounted on both rear axles have good traction, it is desirable for the interaxle differential to be able to function as a differential, so that the rotational speed of the output shafts may vary relative to one another. Under adverse conditions, however, such as a slippery road surface of ice, snow, or mud, when one of the rear axles may lose traction while the other rear axle has good traction, it is desirable to lock out the interaxle differential so that the pinion shaft and the input shaft rotate at a fixed speed ratio and commonly at the same speed. Where wheel slip occurs, torque from the vehicle drive shaft may thereby be transferred to a non-slipping axle for improved vehicle traction.
Various mechanisms, both manual and automatic, for locking out the interaxle differential are known. Earlier tandem rear axle assemblies relied on manual controls for this purpose. More recently, automatic differential lockout mechanisms have come into use. Such automatic mechanisms commonly include a toothed clutch or dog clutch having two relatively axially slidable members which can be brought into or out of engagement with each other, and a clutch actuator assembly including one or more sensors for sensing differential rotation between axles and for actuating the clutch to thereby lock out the interaxle differential. One such condition is a difference in rotational speeds of the two rear axles, which may occur, for example, when one axle is spinning (on snow or ice, for example) while the other axle has traction.
One problem with tandem rear axle assemblies which include a toothed clutch as described above is that they are not smooth in operation, whether the clutch is manually or automatically actuated. Additionally, known differential lockout mechanisms have not provided smooth automatic control to give precise traction control.